1. Field of Invention
The present invention pertains to methods and apparatus for purifying ammonia to obtain an ultra-pure ammonia product, particularly for use in semiconductor manufacturing and related applications.
2. Related Art
Ammonia is typically utilized in the manufacture of electronic devices such as semiconductor chips, light emitting diodes (LEDs) and flat panel display devices, as a source of nitrogen for depositing thin nitride films during formation of the particular device (e.g., tantalum nitride, titanium nitride, gallium nitride and silicon nitride). For example, in the production of semiconductor chips with copper interconnections, a barrier layer of a metal nitride is typically formed by the reaction of a metal precursor (e.g., titanium, tantalum or silicon) with ammonia gas to prevent the migration of copper into regions adjacent the interconnections.
Ammonia gas must be substantially and extremely pure when used in such manufacturing processes to prevent impurities from forming within the metal nitride film layers deposited on the devices being manufactured. Even trace amounts of impurities present in the deposited films can deteriorate film quality and performance. For example, trace amounts of oxygen in ammonia gas (e.g., 10 parts per billion (ppb) or less) used for manufacture of LED devices can cause lattice defects and are detrimental to band gap properties that are desirable for LED performance. Trace impurities of other components in the ammonia gas, such as moisture or water, hydrocarbons and carbon oxides (e.g., carbon dioxide), are also detrimental and can lead to degradation of the final product if present in the nitride films being formed. Further, certain impurities like moisture are very difficult to remove from ammonia gas to a sub parts per million (ppm) level.
At present, commercially available purifiers capable of removing moisture from ammonia at ppb levels are often very expensive and are not suitable for large scale processing flows of about 100 liters per minute (lpm) or greater. As consumer demand for high technology products involving semiconductor, LED and flat panel manufacturing processes increases, so too will the need for ultra-high purity of ammonia in multi-tonnage quantities and that can be produced in an efficient and economical manner. Ammonia purification systems must be capable of tolerating variability of incoming crude or commercial grade ammonia streams for processing and of reducing a variety of impurities without generating secondary impurities. Such systems should further include adsorbent materials that can be repeatedly regenerated to effectively remove moisture and/or other contaminants at sub ppm levels. Further, the handling of ammonia including transfilling of tanks or cylinders must be carried out carefully to prevent or substantially minimize the potential for metallic contamination of the purified ammonia.
Accordingly, there exists a need to provide a system capable of providing ammonia at ultra-high purity levels and in large (e.g., multi-tonnage) quantities while being both efficient and economical.